Chapter 11Phase Transformations
Time Temperature TransformationsContinuous Cooling Transformations
Precipitation Hardening
Slides contain Copyrighted material from Callister
text, 2nd
ed.
Chapter 11 – Phase Transformations•
Phase Diagram tells us….? •
Solid state transformations ‐
diffusion dependent. (T and t)•
Phase transformation study allows prediction of reasonably
attainable microstructure –
Properties depend on microstructure
Phase transformation rate ‐
Temperature dependence
fastslow
When is transformation complete?When does transformation stop?Rate vs. % transformed
T1
T2
Final Microstructure depends on T of transformation1.
Solidification
2.
Solid state precipitation
N&
rate
T
TmTΔ
Why this shape?
Nucleation Rate
By Definition:Transformation rate
5.0
1t
ln(time) at T
Frac
tion tran
sformed
Phase transformation rate ‐
Time dependence (kinetics)
Transformation rate changes over
time –
driving force diminishes
Why? ‐
Consider case of precipitation of
2nd
phase from solid solution
k and
n
are constant for given
T and transformation
)exp(1 nkty −−=Curve described by Avrami
eqn.
Transformation rate: Time and Temp effect
)exp(1 nkty −−=
5.0t
Transformation rate =
5.0
1t
Percen
t trans
form
ed
Temp(0C) Transformation rate (min‐1)135 1/988 1/210
Fe‐C TTT diagrams
• Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)
• Eutectoid transf. (Fe-C System):
α ferrite
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6
.7
L
γ austenite
γ+L
γ+Fe3C
Fe3C
cementiteα+Fe3C
α+γ
L+Fe3C
(Fe) Co, wt% C
Eutectoid:
0.7
7
727°C
T(°C)
ΔT
0.0
22
γ ⇒ α +Fe3C0.77wt%C
0.022wt%C6.7wt%C
Undercooling by ΔT: Ttransf. < 727ºC
Equil. cooling: Ttransf. = 727ºC
Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)
TRANSFORMATIONS & UNDERCOOLING
•
Growth of pearlite from austenite:
Adapted from Fig. 10.3, Callister 6e.
EUTECTOID TRANSFORMATION RATE ~ ΔT
γαααα
α
α
pearlite growth direction
Austenite (γ) grain boundary
cementite (Fe3C)
ferrite (α)
γ
Diffusive flow of C needed
α
αγ γ
α
% Pearlite
0
50
100
Nucleation regime
Growth regime
log (time)t50
Nucleation rate increases w/ ΔT
Growth rate increases w/ Tat given temp, T1
•
Reaction rate is a result of nucleation and growthof crystals.
•
Examples:
Nucleation rate high
T just below TE T moderately below TE T way below TENucleation rate low
Growth rate high
γ γ γ
pearlite colony
Nucleation rate med .Growth rate med. Growth rate low
NUCLEATION AND GROWTH
675°C (ΔT smaller)
1 10 102 103time (s)
0
50
100
y (%
pe
arl
ite
) 0
50
100
600°C (ΔT larger)
650°C
% a
ust
en
ite
At 15% transformed.
•
Fe-C system, Co
= 0.77wt%C•
Transformation at T = 675C.
Adapted from Fig. 10.4,Callister 6e. (Fig. 10.4 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)
ISOTHERMAL TRANSFORMATION DIAGRAMS
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727°C)Austenite (unstable)
Pearlite
T(°C)
100
50
01 102 104
T=675°C
y,
% t
ran
sfo
rme
d
time (s)
time (s)
isothermal transformation at 675°C
675°C (ΔT smaller)
1 10 102 103time (s)
0
50
100
y (%
pe
arl
ite
) 0
50
100
600°C (ΔT larger)
650°C
% a
ust
en
ite
•
Eutectoid composition, Co
= 0.77wt%C•
Begin at T > 727C
•
Rapidly cool to 625C and hold isothermally.
Adapted from Fig. 10.5,Callister 6e. (Fig. 10.5 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)
EX: COOLING HISTORY Fe‐C SYSTEM
1 10 102 103 104 105 time (s)
500
600
700
T(°C)
γ γ
γγγγ
γ
Austenite (stable)
Pearlite
0%pearlite
100%
50%
TE (727°C)
10
μm
- Smaller ΔT: colonies are larger
- Larger ΔT: colonies are smaller
• Ttransf
just below TE
Two cases:• Ttransf
well below TE
PEARLITE MORPHOLOGY
interface energyHigherT: diffusion is fasterPearlite
is coarserLower T: diffusion is slowerPearlite
is finer.
•
Bainite:--α
lathes (strips) with longrods of Fe3C
--diffusion controlled.•
Isothermal Transf. Diagram
Adapted from Fig. 10.9,Callister 6e.(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)
(Adapted from Fig. 10.8, Callister, 6e. (Fig. 10.8 from Metals Handbook, 8th ed.,Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)
Fe3C
(cementite)
5 μm
α (ferrite)
OTHER TRANSFORMATION PRODUCTS: Fe‐C
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
100% bainite
pearlite/bainite boundary100% pearlite
A
A
Micrograph scale difference
60 μm
α (ferrite)
Fe3C
(cementite)
•
Spheroidite:--α
crystals with spherical Fe3C--diffusion dependent.--heat bainite or pearlite for long times--reduces interfacial area (driving force)
•
Isothermal Transf. Diagram
Adapted from Fig. 10.9,Callister 6e.(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)
(Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United States Steel Corporation, 1971.)
Fe‐C SYSTEM ‐
Spheroidite
10 103 105time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
A
A
Spheroidite100% spheroidite
100% spheroidite
Fe‐C SYSTEM: Martensitic
transformation
•
Occurs when Austenite rapidly
cooled to room temp.•
Supersaturated solid solution–
C stays in interstitial sites–
No diffusion during transformation–
Very unstable
xx x
xx
xpotential C atom sites
Fe atom sites
(involves single atom jumps)
•
Martensite:--γ(FCC) to Martensite (BCT)
Unique properties – very brittle
•
Martensite:--γ(FCC) to Martensite (BCT)
Adapted from Fig. 10.13, Callister 6e.
(Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United States Steel Corporation.)
•
Isothermal Transf. Diagram
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
S
M + AM + A
M + A
0%50%90%
Martentite needlesAustenite
60
μm
•
γ
to M transformation..
--
is rapid!--
% transf. depends on T only.--
remaining material is retained Austenite
Fe‐C SYSTEM ‐
Martensite
Adapted from Fig. 10.15, Callister 6e.
COOLING EX: Fe‐C SYSTEM (1)• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
0%
100%50%
A
S
M + AM + AM + A
0%50%90%
100% Bainite
A
100%A 100%B
Case I
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
0% 100%50%
A
S
M + AM + AM + A
0%50%90%
M + trace of A
A
100%A
Case II
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
Adapted from Fig. 10.15, Callister 6e.
COOLING EX: Fe‐C SYSTEM (2)
Adapted from Fig. 10.15, Callister 6e.
COOLING EX: Fe‐C SYSTEM (3)
Rapid cool to:
350°C
250°C
650°C
Hold for:
104s
102s
20s
Rapid cool to:
Troom
Troom
400°C
Hold for:
104s
102s
103s
Rapid cool to:
Troom
Troom
Troom
• Co = Ceutectoid• Three histories...
time (s)10 103 10510-1
400
600
800T(°C)
Austenite (stable)
200
P
B
0%
100%50%
A
S
M + AM + AM + A
0%50%90%
50%P, 50%B
A
50%P, 50%A
50%P, 50%A
100%A
50%P, 50%B
Case III
Austenite (γ)
Bainite (α + Fe3C plates/needles)
Pearlite (α + Fe3C layers + a
proeutectoid phase)
Martensite (BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine
Fe3C particles)
slow cool
moderate cool
rapid quench
reheat
Str
en
gth
Du
cti
lityMartensite
T Martensite bainite
fine pearlite coarse pearlite
spheroidite
General Trends
SUMMARY: Fe‐C Processing
Spheroidite
?
TTT for Hypo‐eutectoid composition
Fe‐C: Continuous Cooling Transformations
•
reduces brittleness of martensite,
•
reduces internal stress caused by quenching.
Adapted from Fig. 10.24, Callister 6e. (Fig. 10.24 copyright by United States Steel Corporation, 1971.)
Adapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig. furnished courtesy of Republic Steel Corporation.)
TEMPERING MARTENSITE
• decreases TS, YS but increases %AR
YS(MPa)TS(MPa)
800
1000
1200
1400
1600
1800
30
4050
60
200 400 600Tempering T (°C)
%AR
TS
YS
%AR
9 μ
m
• produces extremely small Fe3C particles surrounded by α.
•
Particles impede dislocations.
•
Ex: Al-Cu system
• Procedure:--Pt A: solution heat treat
(get α
solid solution)--Pt B: quench to room temp.--Pt C: reheat to nucleate
small θ
crystals withinα
crystals.•
Other precipitationsystems:• Cu-Be• Cu-Sn• Mg-Al
Pt A (sol’n heat treat)
Pt B
Pt C (precipitate θ)
Temp.
Time
Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)
Adapted from Fig. 11.20, Callister 6e.
PRECIPITATION HARDENING
300
400
500
600
700
0 10 20 30 40 50wt%Cu(Al)
Lα+Lα
α+θθ
θ+L
T(°C)
A
B
C
composition range needed for precipitation hardening
CuAl
Preview of Mechanical Properties
Precipitation hardening for temp, T‐
Longer time, larger ppts‐
Different curve for each T
Al ‐
Cu
Preview of Mechanical Properties
All pearlite
is in the form of fine pearlite.
Preview of Mechanical Propertiesha
rdne
ss
Duc
tility
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